This invention relates to shoring devices of the type used for stabilizing collapsed structures such as trenches or buildings. Particularly, these shoring devices are useful in rescue and safety applications for stabilizing a collapsed or unstable structure in order to safely and quickly allow rescue personnel, such as firemen, to reach and evacuate victims who may be injured or trapped inside. The shoring device of the present invention may also be used to maintain stability in the collapsed structure temporarily while the structure is repaired or reconstructed. Also, the shoring device disclosed herein may be used for stabilization of trenches that are dug for the laying of pipe, cable, etc. In particular, this invention is directed to an infinitely adjustable hydraulic/pneumatic self locking shoring strut for use in the above-mentioned applications.
Shoring devices for preventing the collapse of unstable structures to permit safe passage of rescue or construction personnel are well known. These struts have taken on many different designs and features over the years, utilizing various techniques to accomplish the goal of safer and easier to use equipment. One of these methods involves the use of a screw-type expander mechanism, the operation of which is well-known in the art. U.S. Pat. No. 3,393,521 discloses a shoring strut of this type wherein an externally threaded shaft is disposed in a housing within a threaded collar and has a load bearing surface on the distal end. In order to operate the strut, the collar is rotated with respect to the shaft forcing the shaft outward from the housing.
The struts of the type disclosed in U.S. Pat. No. 3,393,521 are not without problems. First, if pressure is being applied to the distal end of the strut, rotational force proportional to the pressure exerted on the end of the strut is required to extend the strut. Thus, if the screw type strut is extended by human power, there are situations where the strut could not be extended to the required length due to the incapability of the human operator to generate enough torque to extend the strut. Another problem arises in that although the friction between the threads on the strut and the threads on the collar is sufficient to prevent the strut from retracting due to the pressure on the distal end to some extent, generally a mechanical stop, such as a pin or a detent mechanism, is required to insure that the strut does not self retract, thereby threatening the safety of the operators. Two features that are generally considered highly desirable in a shoring strut, namely a self locking characteristic and infinite adjustability, have not yet been simply and reliably achieved with the use of mechanical stop mechanisms. Furthermore, in the case of pin type stops, the operator is required to perform the delicate and potentially dangerous task of aligning the holes for insertion of the pin while simultaneously maintaining the position of the strut. A self locking feature is desirable because the strut can be lowered into the required location and extended remotely without the operator having to enter the unsafe area. While struts utilizing a detent mechanism can be self locking, thereby avoiding some of the problems associated with pin type stops, they also generally are not infinitely adjustable. Furthermore, the reliability of these detent type stops can be questionable, being prone to slippage or failure thereby jeopardizing the safety of the operator.
Struts have been developed that utilize hydraulic forces to extend the strut. U.S. Pat. No. 3,404,533 discloses a shoring strut of this type which utilizes a linkage arrangement coupling hydraulic devices to a pair of vertical shoring members in a pivotal manner. A locking means is also provided to lock the strut in place once it is properly situated. U.S. Pat. No. 4,682,914 discloses a similar strut of this type comprising a piston disposed within a cylinder which is hydraulically operated and which has a spring biasing means to contract the strut in the absence of hydraulic pressure. However, the locking force of the '914 patent is provided by the continued application of the hydraulic pressure, and has no independent locking means. U.S. Pat. No. 4,787,781 discloses a trench shoring device utilizing a hydraulic piston and a locking device. The piston is threaded and a nut is provided which can be rotated down the piston shaft after the strut is extended to engage the outer piston cylinder, thereby preventing piston contraction. U.S. Pat. No. 5,499,890 discloses a trench shoring device with a locking mechanism which uses a piston in a cylinder that is reciprocated hydraulically. The locking mechanism consists of a pair of cam lobes which are rotated towards the piston thereby grabbing and locking said piston in place when corresponding hydraulic securing pins are activated.
While the use of hydraulically actuated pistons and mechanical locking systems as described above is responsive to some problems found in prior shoring struts, several other areas needed to be addressed. First, while the use of hydraulic based systems is preferred over rotational screw-type struts, the hydraulic pumps required to expand struts under heavy loads need to be quite large and powerful. Therefore, they tend to be complicated and expensive. Furthermore, especially in rescue situations, many struts may be required to stabilize the collapsed structure. Therefore, in an all hydraulic system, a large reservoir is necessary to insure that there is enough hydraulic fluid available to adequately expand all of the required struts. Furthermore, when the struts are retracted, the same fluid capacity is necessary to receive the fluid from the struts. Also, the hoses and connections required for these hydraulic systems can be difficult to handle. As can easily be imagined, a system such as this is not only large and cumbersome, but it can become quite messy, especially when leaks begin to develop in the system.
Pneumatic systems have many advantages over hydraulic systems. While reliable heavy duty pneumatic compressors can be obtained relatively inexpensively, pneumatic systems can also be actuated by compressed air that is fed from bottles or tanks. This feature is particularly useful in rescue situations where a reliable power source may be unavailable or the operating space may be strictly confined or not easily accessible. In such cases, rescue personnel may use air from readily available breathing bottles in order to expand the struts, increasing the speed with which they may reach endangered victims. Obviously, time is critical in these potential life and death situations, and any time savings that may be obtained are highly desirable. Also, as mentioned above, the mess and complications found in hydraulic set-ups are not found in similar capacity pneumatic systems. However, all pneumatic systems have a few inherent problems. The application of high pressure air has been known to drive the piston completely out of the strut. Furthermore, the advantages of a hydraulic system, particularly the superior damping qualities, are not present in an all pneumatic system.
Thus, there is a need for an infinitely adjustable self locking shoring strut which utilizes the advantages of both pneumatic and hydraulic systems, is of relatively simple construction, is safe and reliable, and is inexpensive to fabricate.